1. Field of the Invention
This invention relates to an optical tomography system for obtaining an optical tomographic image by measurement of OCT (optical coherence tomography).
2. Description of the Related Art
As a method of obtaining a tomographic image of an object of measurement such as living tissue, there has been proposed, in addition to an ultrasonic tomography system, a method where OCT (optical coherence tomography) is measured as disclosed in Japanese Unexamined Patent Publication Nos. 6(1994)-165784 and 2003-139688. In the OCT measurement, a phenomenon that interference light is detected when the optical paths of the measuring light and the reflected light conform to the optical path of the reference light in length is used. That is, in this method, low coherence light emitted from a light source is divided into measuring light and reference light and the measuring light is projected onto the object of measurement, while the reflected light from the object of measurement is led to a mixing means. The reference light is led to the mixing means after its optical path length is changed in order to change the depth of measurement in the object. By the mixing means, the reflected light and the reference light are superposed one on another, and interference light due to the superposition is detected by, for instance, heterodyne detection.
In the above OCT system, a tomographic image is obtained by sweeping the optical path length of the reference light, thereby changing the measuring position (the depth of measurement) in the object. This technique is generally referred to as “TD-OCT (time domain OCT)”. More specifically, in the optical path length changing mechanism for the reference light disclosed in Japanese Unexamined Patent Publication No. 6(1994)-165784, an optical system which collects the reference light emitted from the optical fiber on a mirror is provided and the optical path length is adjusted by moving only the mirror in the direction of the beam axis of the reference light. Further, in the optical path length changing mechanism for the reference light disclosed in Japanese Unexamined Patent Publication No. 2003-139688, the reference light emitted from the optical fiber is turned to parallel light by a parallel lens, and the reference light in the parallel light is collected and caused to enter the optical fiber again by an optical path length adjusting lens, and the optical path length adjusting lens is moved back and forth in the direction of the beam axis of the reference light to adjust the optical path length.
Whereas, as a system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an SS-OCT (swept source OCT) system where interference light is detected while the frequency of the light emitted from the light source is changed with time. In the SS-OCT system, an interferogram interference intensity signal is obtained without changing the optical path length by sweeping the frequency of the laser beam emitted from the light source to cause the reflected light and the reference light to interfere with each other by the use of a Michelson interferometer. Then a tomographic image is generated by carrying out a Fourier analysis on the interferogram signal in the region of an optical frequency.
Whereas, as a system for rapidly obtaining a tomographic image without sweeping the optical path length of the reference light, there has been proposed an optical tomography method of obtaining an optical tomographic image by measurement of SD-OCT (spectral domain OCT). In the SD-OCT system, a tomographic image is formed without scanning in the direction of depth, by dividing broad band, low coherence light into measuring light and reference light by the use of a Michelson interferometer, and carrying out a Fourier analysis on each channeled spectrum obtained by decomposing the interference light of the reflected light, which returns when projecting the measuring light onto the object, and the reference light into frequency components.
In the SD-OCT system and the SS-OCT system described above, it is still necessary to adjust the optical path difference within an interference distance within which the measuring light and the reference light can interfere with each other, that is, to adjust the optical path length of the reference light (or the measuring light) within a measurable range within which a tomographic image can be obtained. As a means for adjusting the optical path length, it is conceivable to employ the optical path length adjusting mechanism disclosed in Japanese Unexamined Patent Publication Nos. 6(1994)-165784 and 2003-139688.
However, when only a mirror or a lens is moved as in the optical path length adjusting mechanism disclosed in Japanese Unexamined Patent Publication No. 6(1994)-165784, there is involved a problem that a stable intensity of the reflected light cannot be obtained when the measuring position is scanned in the direction of depth since light radiated from the optical fiber cannot be collected on the mirror and reflected light from the mirror which enters the optical fiber is reduced in amount.
Further, when an optical path length adjusting lens is moved back and forth in the direction of the optical axis of the reference light in order to adjust the optical path length, the position of focus of the optical path length adjusting lens is also moved and there is involved a problem that a stable intensity of the interference light cannot be obtained when the measuring position is scanned in the direction of depth since reference light radiated from the optical path changing lens which enters the optical fiber is reduced in amount.
That is, when reference light propagating an optical fiber is taken out to be adjusted in its optical path length and is returned to the optical fiber, it is necessary to cause the light to enter the core of the optical fiber which is several μm to ten μm in its diameter. Accordingly, a stable intensity of the interference light cannot be obtained and the quality of the image can deteriorate when the measuring position is scanned in the direction of depth since the amount of reference light which enters the optical fiber when light is deviated from the core of the optical fiber or the beam diameter is increased upon adjustment of the optical path length disclosed in Japanese Unexamined Patent Publication Nos. 6(1994)-165784 and 2003-139688.
Also when the optical path length adjusting mechanisms disclosed in Japanese Unexamined Patent Publication Nos. 6(1994)-165784 and 2003-139688 are employed in the SS-OCT system and the SD-OCT system, there is a problem that the intensity of interference light is reduced upon adjustment of the optical path length and the quality of the image can deteriorate.